The present invention relates to a method for depositing materials on the sea bed in accordance with a given layout configuration and to apparatus for performing the said method.
More specifically, the present invention relates to a method for depositing materials such as gravel or rocks on a pipeline placed on the sea bed, whereby said pipeline can for example be a gas or oil pipeline, a cable, etc.
As is known, problems are caused by the existence of cables or pipes on the sea bed. In addition to the necessity to immobilise them, they must be protected so that they will not be accidentally damaged by trawls or anchors of ships or any other object dragged along the sea bed. Conversely, to prevent damage to fishing tackle, such as trawls, these cables or pipes must not be left in an exposed an unsecured manner on the sea bed. Therefore, said cables or pipes have to be buried, which involves the digging out of a trench in the sediment on the sea bed in order to place the cables or pipes therein, whereby when the sea bed is rocky, the trenches have to be made in this rock, at least in the case of cables. When the sediment of the sea bed has a limited cohesion, the trench is generally made by using jets of water. The cable or pipe is then covered naturally by sediment.
When the sea bed is hard and there is no possibility of digging out a trench or for traversing basins on the bottom of the ocean, the aforementioned methods cannot be used. In addition, ocean currents can lead to the exposure of buried cables or pipes, making it necessary to place protective materials thereon.
When the deposits have to be made up to depths of 100 meters, such materials are discharged from the surface. If it is desired to cover the complete tube which can in particular be a large pipeline, it is necessary to deposit a minimum thickness of about 1 meter thereon and due to the spreading out of the material from its drop point it is necessary to use 20 to 100 times more material than would be necessary if deposition was very accurate.
Installations for depositing rocks on a pipeline resting on the sea bed are also known in which there is a surface ship, an autonomous vehicle which is mechanically obliged to follow the line of the pipeline and a discharge pipe suspended on the ship. The vehicle carries a hopper and a ring integral with the hopper mechanically forces the lower end of the pipe to remain above the hopper. With such a system, it is obvious that all the forces applied to the pipe and in particular to its free lowe end are integrally transmitted to the vehicle, due to the fact that there is a mechanical connection between vehicle and pipe, with no real possibility of absorbing these forces other than by the vehicle. The forces are essentially due to the inertia of the ship and/or the pipe, either as a result of the effect of the swell or due to position changes to the pipe. Moreover, the vertical amplitude of pounding makes it necessary to position the ring sufficiently high above the vehicle hopper. It is clear that unless the height of the pipe is reduced (for example to below 50 m) the forces applied by the pipe to the vehicle are very considerable with such a system, even if the pipe moves very slowly. To maintain the vertical stability of the vehicle, it is then necessary for the latter to have a very large and very heavy impaling system. This will render its construction more complex and costly. In addition, the autonomous propulsions means must be much more powerful and their bearing surfaces much larger, which increases with weight. Thus, the forces applied by the lower end of the pipe to the vehicle may make it possible in practice to obtain a good stability of said vehicle. However, even if the vehicle is made sufficiently stable to prevent it tipping over through providing a large impaling system, said forces are liable to displace the vehicle from its trajectory.
It is also known that it is possible to control the position of a ship by so-called dynamic positioning. This makes it possible to obtain an extremely accurate positioning of the ship leading to a positioning relative to the sea bed with an accuracy of at the best approximately 1% of the depth. Thus, to solve the problem of depositing rocks on a pipeline by means of a self-propelled vehicle, said dynamic positioning of a ship can only be used for limited depths, which must certainly be below 100 meters.